There isn't really any inherent advantage to building from source in and of itself, the real cool stuff comes from being able to customise your programs and your OS however you want it. For example, don't want any program you install to be compiled with pulseaudio support? Add "-pulseaudio" to your USE flags in your make.conf. Want your programs to be built with SSL support? Add "ssl" to your USE flags. You want systemd? Install it! You don't? Well then don't, it's not like you're forced to build packages with features that require systemd, just add "-systemd" to your USE flags and it won't be a problem.

There's obviously more complicated stuff you can do with Gentoo, but for day-to-day usage, being able to mix and match features without having to edit and shell scripts or even know what build system the package uses is great.

I haven't actually used CRUX, but I don't know of any killer features it provides over Gentoo, whereas Gentoo has USE flags, a larger community, lots more packages and overlays and generally more up-to-date packages (although maybe that's a negative, since Gentoo isn't stable).

Don't take it from me, though, you should try both out if you get the time and make your own decision.